1. Field of the Invention
The present invention relates to a communication apparatus and a communication calibration method, and in particular, relates to a communication apparatus and a communication calibration method that perform a calibration for compensating for characteristic differences among elements of a plurality of antennas in the communication apparatus having the plurality of antennas.
2. Description of the Related Art
A wireless network has gained attention as a system that allows people free from wiring in a cable communication system in related art. IEEE (The Institute of Electrical and Electronics Engineers) 802.11 can be cited as a standard for a wireless network.
In IEEE 802.11a/g, for example, the OFDM (Orthogonal Frequency Division Multiplexing) modulation method, which is a kind of multicarrier method, is adopted as a standard for wireless LAN. According to the OFDM modulation method, transmission data is transmitted by being distributed to a plurality of carriers to which frequencies orthogonal to each other are set in the OFDM modulation method and the band of each carrier becomes narrow, resulting in characteristics of very high utilization efficiency of frequency and being resistant to frequency selective fading disturbances.
While the standard of IEEE 802.11a supports a modulation method reaching a communication speed of up to 54 Mbps, a wireless communication standard capable of realizing a still higher bit rate has been sought after. A multi-antenna technology in which a communication instrument has a plurality of antennas can be cited as a wireless communication technology to realize wireless data transmission of high throughput.
An adaptive array antenna in which a plurality of antennas are arranged in an array form so that directivity can be dynamically changed adjusting to radio environment changes is widely known as an example of multi-antenna technology. The adaptive array antenna supports communication by controlling the gain of each antenna element to obtain appropriate antenna directivity in transmission/reception. That is, signals received by each antenna element constituting the adaptive array antenna are synthesized after each signal being weighted by multiplication of an appropriate weighting factor to control a receiving directivity pattern as a whole array antenna. Moreover, a transmitting directivity pattern as a whole array antenna is controlled by multiplying each transmission signal by an appropriate weighting factor for each antenna element before being transmitted from each antenna element.
As another example of wireless communication technology using a multi-antenna, MIMO (Multi-Input Multi-Output) communication has received attention. This is a communication method in which a plurality of antenna elements is provided both on the transmitter side and receiver side to realize a plurality of spatially multiplexed MIMO channels. On the transmitter side, transmission data is transmitted by being distributed to a plurality of streams using the plurality of antennas, and the receiver can extract a signal of each stream without cross talk by performing signal processing on spatial signals received by the plurality of antennas. In IEEE 802.11a/n, for example, the MIMO-OFDM method using OFDM for primary modulation is adopted. According to the MIMO communication method, improvement of the communication speed can be achieved by expanding transmission capacity in accordance with the number of antennas without increasing the frequency band.
The SVD-MIMO method using singular value decomposition (SVD) of a channel function H is known as a typical example of the MIMO communication method. In SVD-MIMO transmission, UDVH is determined by performing singular value decomposition of a numeric matrix having channel information corresponding to each antenna pair as an element, that is, a channel information matrix H to provide a matrix V as an antenna weighting factor matrix on the transmission side and (UD)H as an antenna weighting factor matrix on the reception side (U and V are unitary matrices and D is a diagonal matrix). Accordingly, each MIMO channel is represented as a diagonal matrix D having a square root of each singular value (λi) as an element. That is, a plurality of transmission paths that are spatially orthogonal and multiplexed and theoretically independent is realized, and a sequence of a plurality of original signals can be extracted on the receiver side without cross talk at all so that the highest performance is theoretically achieved.
In an adaptive array antenna, however, a directivity pattern on the transmission system and that on the reception system do not match due to fluctuations in characteristics of each element constituting the transmission/reception system and line length (hereinafter, referred to also as “irreversibility”). Therefore, even if weighting factors calculated for reception are used during transmission unchanged, a receiving directivity pattern and a transmitting directivity pattern will not match.
To compensate for such irreversibility of directivity patterns, correction value calculation processing called a calibration is performed in the adaptive array antenna in advance and a receiving directivity pattern and a transmitting directivity pattern are made to match by correcting the weighting factors using correction values obtained during the calibration. The calibration method is roughly divided into a calibration inside apparatus (self calibration) and that outside apparatus.
The calibration outside apparatus uses, in addition to a communication apparatus having an adaptive array antenna (referred to as an apparatus A), a communication apparatus for calibration (referred to as an apparatus B). After receiving a reference signal transmitted from each antenna element of the adaptive array antenna included in the apparatus A, the apparatus B returns the received reference signal to the apparatus A as feedback. Then, the apparatus A having the adaptive array antenna detects fluctuations in characteristics of each transmission/reception system including antennas and propagation path by comparing the reference signal turned back from the apparatus B and the original reference signal to correct fluctuations.
The calibration inside apparatus, on the other hand, causes a reception system to turn back a reference signal generated by each transmission system of the communication apparatus, compares the reference signal turned back by the reception system and the original reference signal, and detects fluctuations in characteristics of each transmission/reception system including antennas and propagation path to correct fluctuations.
In the calibration outside apparatus, it is difficult to transmit data during feedback of a reference signal from the apparatus B to the apparatus A and thus, throughput during communication from the apparatus B to the apparatus A is degraded. If feedback from the apparatus B to the apparatus A takes time, the propagation environment may change in the meantime. That is, there is an issue that the calibration outside apparatus is subject to a surrounding environment and the signal-to-noise ratio (SN ratio).
The calibration inside apparatus, on the other hand, is resistant to a surrounding environment and the signal-to-noise ratio (SN ratio) when compared with the calibration outside apparatus because the calibration is completed within an apparatus. A technology to perform a calibration using the calibration inside apparatus is disclosed, for example, in Japanese Patent Application Laid-Open No. 2007-116489.